Abstract: An intense light source, preferably of high monochromaticity, having its beam modulated at a frequency f.sub.o through a wavelength range including an absorption line of a species to be detected, energizes a chamber containing a sample of the species. The modulation frequency may also be a multiple or sub-multiple of the natural resonant accoustic frequency of the sample chamber. A microphone in the sample chamber provides an output signal proportional to the concentration of the species. The radiation beam is further incident upon a calibration chamber containing a predetermined concentration of the species. A microphone in the calibration chamber provides a calibration signal for comparison with the sample signal to determine the concentration of the species in the sample cell. Means are provided for adjusting the center of the wavelength modulation range so that it bears a known relation to the center of the species absorption line.

Abstract: One or more lasers direct one or more beams of radiant energy upon a nonlinear crystal having a sequence of rotational twin planes parallel to each other and perpendicular to the incident beams to produce an output beam at predetermined frequency at relatively high power level that may be detected or used in a suitable utilization device. Preferably the twin planes in the crystal are separated from each other by uniform spacing related to the coherence length for the nonlinear process being employed so that the power enhancement is proportional to the number of twin planes squared.

Abstract: A spectrophotometric method is disclosed for measuring the heating value of a mixture of substances commonly comprising a gaseous fuel. The method is based on the recognition that within certain wavebands the mixture exhibits a correlation between heating value and the strength of radiant energy absorption. A spectrophotometric apparatus is disclosed which is adapted to permit the continuous observation of the heating value of the mixture.

Abstract: An intense light source, preferably of high monochromaticity, having its beam modulated at a frequency corresponding to an acoustical resonant frequency of a sample chamber energizes the sample chamber along its axis. A photoelectric detector provides a signal related to this modulation for comparison with an acoustical signal provided by a microphone in the sample chamber. Means are provided for adjusting the modulation frequency so that it corresponds to the acoustical resonant frequency of the sample chamber as determined by a maximum in the amplitude of the amplified acoustical signal. An integrator may respond to the amplified acoustical signal to provide an indication of the energy absorbed by the medium in the sample chamber at the light wavelength and thereby the concentration of certain species in the sample chamber.